Meteorological spring is upon us, and it is time again to take a look at the spring tornado forecast. Are we going to see a repeat of the last two years with low tornado numbers? Or are we finally going to turn things around in 2015?

Not much is expected in the way of tornado activity through the first chunk of March. It’s early for much in most years anyway, but we do usually see an uptick as the month goes.

Last month, we showed you the basic ingredients needed for tornadoes to form. In part two of our series, we will take a look at the atmosphere in the horizontal to figure out where tornadic storms may form. This almost always means finding a boundary. Some boundaries are harder to find than others.

Most boundaries represent some sort of converging wind shift at the surface and in the lower levels of the atmosphere, which forces two different types of air masses together. When this happens, the converging air has no where to go but up. Rising air helps initiate cloud development. Under the right circumstances, these clouds can develop into tornadic thunderstorms.

The low-level winds along a boundary can provide storms with a continuous feed of warm, moist air, and can also enhance a storm’s low-level rotation in some cases.

NOTE: This is NOT a guide to storm chasing. It is meant for forecasting purposes only. The series reflects what is needed for a typical supercell tornado, and does not necessarily match what is needed for landspouts or waterspouts.

Tornadoes sweep through the Pilger, NE area in June 2014. (Aaron Rigsby via Flickr)

This is the first of a three part series that covers the basics on how to forecast tornadoes. In this post I’ll concentrate on the general necessary ingredients needed for a tornado to form.

Part two will examine the surface and low-level features in the horizontal layers (mostly meaning boundaries), and part three will discuss what you can look for that might hurt tornado development.

NOTE: This is NOT a guide to storm chasing. It is meant for forecasting purposes only. The series reflects what is needed for a typical supercell tornado, and does not necessarily match what is needed for landspouts or waterspouts.

Wind Shear

Wind shear creates spin. Tornadoes spin. So naturally, you want more wind shear to create more spin. However, there are a lot of subtleties to how the wind changes with height that can drastically alter how favorable a setup is for producing tornadoes.

In the most general sense, you want wind speeds to increase with height, and you want them to veer — change direction — with height. Veering in the northern hemisphere means the wind direction moves clockwise (i.e. south to west to north) as you gain altitude. Wind shear helps shape a thunderstorm’s updraft and gives it rotation. It is the most critical ingredient to creating a tornadic supercell.

It’s also not far removed from specific tracks of prior deadly winter tornadoes. Quick note: In all cases here, winter refers to December-February — meteorological winter, and coinciding with the lowest three months of tornado activity on average.

Though tornado season lasts all year, it’s often hard to remember that in December. The monthly average is only about two dozen twisters across the entire country.

Numbers remain very low through the winter months as moisture-laden air is often hard to come by. Like other cool-season months, the most regular tornado activity during December resides close to the relative warmth of the Gulf of Mexico.

The areas of greatest concentration — spanning mainly parts of the mid-South — are not too different than November. But as with November seeing a contraction of land coverage compared to October, we see additional shrinking of “tornado country” during December.

Often considered part of the “second season” which sometimes occurs in fall, tornado activity is significantly down by October when compared to the yearly peak in spring and early summer.

By October, tornado territory is dwindling with the encroachment south of colder and drier air, plus tropical season peak is behind us.

Outside larger outbreaks that occur at random, a trend of fairly few tornadoes generally closer to the Gulf of Mexico becomes the norm around this time of year. U.S. tornadoes never really fully disappear, they just wane significantly.